This invention relates generally to the growth of semiconducting thin-film materials and relates, more particularly, to the deposition of thin film materials upon heated substrates by pulsed laser techniques.
Pulsed laser deposition (PLD) is a film growth method in which a solid (usually polycrystalline) target is irradiated with a pulsed laser beam. During the initial moments of each laser pulse, atoms are desorbed from the target and a laser-generated plasma is formed adjacent to the target surface. During the latter moments of each pulse, the laser heating of the plasma and its interaction with the target result in a thin near-surface layer of the target being decomposed down to the atomic and molecular level and ejected from the target. This process is known as pulsed laser ablation (PLA). Rapid expansion of the heated plasma causes the ablated material to be ejected nearly perpendicular to the target surface in the form of an energetic beam of electrons, ions, atoms and very simple molecules. Film growth occurs when this energetic ablation beam is deposited on a heated substrate located nearby.
Heretofore, two methods have been used to vary the composition of pulsed laser deposition (PLD) films. In a first known method, several targets are used wherein the targets are of different, fixed chemical compositions. However, the use of this first method is limited in that it permits only films of the same fixed compositions to be grown (e.g., A.sub.o.9 B.sub.o.1, A.sub.0.8 B.sub.0.2, A.sub.0.5, etc.), rather than a continuous range of compositions (e.g., A.sub.1-x B.sub.x wherein "x'is variable).
In the second known method, laser atomic .layer epitaxy (laser ALE) is used in which a single unit-cell thickness of a crystalline film is built up atomic layer-by-atomic layer by alternately ablating targets comprised of the pure elements A and B. An arbitrary composition A.sub.1-x B.sub.x can be grown in this manner, but it is necessary to repeat the process in a unit cell-by-unit cell construction in order to build up a film of useful thickness. Consequently, the laser ALE process is relatively slow and tedious and is impractical for many applications.
To control the useful electrical properties of semiconductor materials, doping is required during the growth of the materials. Doping is carried out by adding a relatively small and carefully controlled amount of a specific, i.e., predetermined, chemical impurity atom to the semiconductor during growth. In order that the doping operation be successful, the dopant (chemical impurity) atom must substitutionally replace a specific one of the elements in the host semiconductor material. For example, "p-type" electrically conducting ZnSe can be produced by replacing a small number of selenium (Se) atoms by nitrogen (N) impurities, forming ZnSe.sub.1-y N.sub.y, wherein the nitrogen concentration "y" typically ranges only from 0.000001 to 0.01 (10.sup.-2). Because of the very small dopant concentration "y", it is extremely difficult to produce laser ablation targets that contain a uniform distribution of exactly the right dopant atom concentration. For the targets to be inexpensive, they must be polycrystalline, but impurity atoms will segregate to the polycrystalline grain boundaries and/or diffuse along them during the heating that typically is required for target fabrication. Furthermore, even if a uniformly doped target could be formed, a new target with different composition would be required in order to change the dopant concentration.
It is an object of the present invention to provide a new and improved method for depositing thin film materials upon heated substrates by pulsed laser techniques.
Another object of the present invention is to provide such a method which circumvents the aforementioned limitation of pulsed laser film growth wherein film composition is limited to that of the ablation target.
Still another object of the present invention is to provide such a method which effects a relatively rapid growth of the film upon the substrate.
Yet another object of the present invention is to provide such a method wherein dopant atoms can be introduced into a semiconductor film with relatively high precision in order to control the film properties.